In reply to dave_59:
Hi Dave,
Thank you very much for the answer. You’re correct that I used “`include my_class.sv” instead of a package. After I changed it to a package, as you suggested, everything looks fine!
The link doesn’t work for me, so I googled it and find it here: http://blogs.mentor.com/verificationhorizons/blog/2010/07/13/package-import-versus-include/. It is a great article.
Thank you again for taking time to answer my question.
Simon